In several types of integrated circuit (IC) packaging technologies, the die is mounted on a portion of the upper surface of a lead frame referred to as the die-attach region, and wire bonds electrically connect pads on the die to areas known as pins on regions of the lead frame surrounding the die-attach region. For example, in a quad-flat no-lead (QFN) package, a lead frame assembly of this type is encapsulated in plastic, and the pins, which are distributed about the four sides of the package, extend to the lower surface of the package to define landing pads that are surface-mountable to a printed circuit board.
As illustrated in FIG. 1, a lead frame assembly 10 of a conventional QFN package includes a lead frame 12 on which a die 14 is mounted. In an exemplary portion of lead frame assembly 10, bond wires 16, 18, 20, 22, 24 and 26 connect die pads 28, 30, 32, 34, 36 and 38 to pins 40, 42, 44, 46, 48 and 50, respectively. Only a portion of the rectangular lead frame assembly 10 is shown in FIG. 1 for purposes of clarity, and a similar arrangement of bond wires, die pads and pins exists on each of the four sides of lead frame assembly 10. Lead frame 12 can also include a ground paddle region 52 that is maintained at a ground potential during operation.
Signal crosstalk is a common problem in IC packages of the above-described type. Such crosstalk is commonly caused by electromagnetic coupling of signals between bond wires that are in close proximity with each other. To inhibit such crosstalk, it is known to interpose ground bond wires between information signal-carrying bond wires susceptible to crosstalk with each other. For example, as illustrated in FIG. 1, bond wire 22 is maintained at ground potential (as indicated by its depiction in solid line as a contrast with broken line) to inhibit crosstalk between the differential pair of bond wires 18 and 20 and the differential pair of bond wires 24 and 26, which carry information signals (as indicated by their depiction in broken line as a contrast with solid line). Note that information signal-carrying bond wires are commonly paired in instances in which signals are differential rather than single-ended. However, the same principle of interposing a ground bond wire between otherwise adjacent information signal-carrying bond wires applies in an instance (not shown) in which signals are single-ended. Bond wires 22 and 16 are disposed on either side of the differential pair of bond wires 18 and 20 to inhibit crosstalk between the differential pair of bond wires 18 and 20 and other bond wires.
A shortcoming of the above-described use of ground bond wires to inhibit crosstalk is that maintaining some of the pins at ground potential limits the number of pins available to carry information signals. Increasing the total number of pins so that more pins are available for ground bond wires increases the outside dimensions of the QFN package. That is, if the total number of pins is increased, each of the four sides must be lengthened to accommodate the additional pins. As it is highly desirable to minimize package size, alternatives to maintaining many pins at ground potential have been developed.
As illustrated in FIG. 2, a lead frame assembly 54 of a conventional QFN package includes a lead frame 56 on which a die 58 is mounted. In an exemplary portion of lead frame assembly 54, so-called “down” bond wires 60, 62 and 64 connect die pads 66, 68 and 70 to the ground paddle region 72 of lead frame 56. As down bond wires 60, 62 and 64 are not connected to pins, all or many of the pins of lead frame 56 (most of which are not shown in FIG. 2 for purposes of clarity) are available to carry information signals. Such information signals are carried by bond wires 74, 76, 78 and 80 which connect die pads 82, 84, 86 and 88 to pins 90, 92, 94 and 96. In a manner similar to that described above with regard to FIG. 1, down bond wire 62, for example, inhibits crosstalk between a differential pair of bond wires 74 and 76 and a differential pair of bond wires 78 and 80.
It has been found that ground down bond wires (FIG. 2) do not inhibit crosstalk to as great an extent as ground pin bond wires (FIG. 1). It has been theorized that because ground down bond wires do not extend all the way from the die pads to the pins, some electromagnetic signal coupling (i.e., crosstalk) can occur in the region where the information signal-carrying pins are immediately adjacent one another. Such crosstalk may not occur to as great an extent in an arrangement (FIG. 1) in which a ground pin bond wire and ground pin are fully interposed between the information signal-carrying bond wires and corresponding pins that are otherwise susceptible to crosstalk. However, as described above, it is desirable to minimize the number of ground pins so that the number of signal pins can be maximized without increasing package dimensions.
Although the above-described conventional bond wire arrangements help minimize crosstalk, crosstalk can remain problematic in some instances. It would be desirable to provide an improved bond wire arrangement that further inhibits crosstalk without providing an excessive number of ground pins.